Surface shape measurement is an important need in many industrial and scientific applications. Typical examples include flatness inspection of road surfaces, quality control of manufactured products such as sawn lumber and rolled metal, and shape determination of complex shaped objects such as logs.
A simple way to measure surface shape profile is to make a series of measurements using a distance sensor while relatively moving the measured object and the sensor in a straight line perpendicular to the measurement direction. FIG. 1(a) schematically shows an example arrangement for a surface shape along a single line. The drawback to this method is that deviations from straight-line motion cause relative displacements in the measurement direction that are indistinguishable from measured surface shape. Thus, very accurate linear motion is essential.
In many cases, accurate linear motion is not practicable. For example, when measuring the surface profile of a highway pavement, it is generally not feasible to provide a separate linear slide for moving the sensor. Instead, the sensor must travel on the irregular pavement that it is measuring.
U.S. Pat. No. 5,280,719 describes an apparatus that uses a large number of equally spaced sensors. The apparatus seeks to identify long surface features by overlapping the sets of measurements made at successive intervals much less than the total spacing of the sensors. However, the large number of sensors that are required creates a large cost and maintenance burden. U.S. Pat. No. 4,573,131 describes a method of using just four sensors to achieve the same objective. The sensors make successive measurements at small intervals of travel. The described method is based on the assumption that successive sensors reaching the same point along the measured line have the same distance from the measured surface. This is typically not a reasonable assumption, and the described method has only limited effectiveness.
The above-mentioned patents disclose methods for measuring surface height profiles along individual lines on the target object. It is also of great practical interest to be able to measure a surface shape map, similar in concept to a topographic map. Such measurements can be achieved by replacing the individual distance sensors shown in FIG. 1 (“point sensors”) by sensors that measure surface height profile at multiple points along lines (“line sensors”). The accumulation of sequential measurements at the various intervals along the measured lines creates a surface shape map. Line sensor measurements are subject to the same influence of deviations from straight-line motion of the measured object during the measurements as point sensors. Canadian Patent 2,297,879 describes a technique where multiple equally spaced parallel lines are measured, with the surface shape identified by overlapping the successive measurements. This approach is a development of that used in U.S. Pat. No. 5,280,719, with multiple line sensors replacing multiple point sensors. Both techniques have similar limitations. Their lengthwise spatial resolution is limited to the distance between the sensors; the use of evenly spaced parallel sensors makes the method unsuitable for achieving finer spatial resolution. When doing measurements using two line sensors, only surface twist can be identified, but not lengthwise shape.
None of the above techniques is well suited to making surface shape measurements with fine lengthwise spatial resolution. They are also not well suited to measuring the surfaces of objects with opposing sides. At best, they can measure each of the sides separately, but they do not provide accurate thickness information. The method described in U.S. Pat. No. 7,003,894 is successful at making fine resolution surface height profile measurements and providing accurate thickness information. The method involves the use of multiple sensors that make sequential surface height measurements. The surface height profile(s) is/are mathematically determined from the measured data by observing that the surface height information appears sequentially in the measured data, while the effects of relative motions occur simultaneously. U.S. Pat. No. 7,003,894 describes several different possible measurement configurations.
The methodology described in U.S. Pat. No. 7,003,894 works well when the number of surface points to be measured is relatively modest, for example, to determine surface shape profiles along one or a small number of chosen lines within the specimen surface. However, the computation becomes very burdensome and time consuming when an entire surface shape map is desired. The present invention provides a different data handling approach that is able to evaluate detailed surface height maps in real time, that is in an interval of time less than required for data acquisition. Furthermore, the method and system operate independent of relative motion, that is, they are independent of movements due to translation, pitch and/or roll that the object may experience when being mapped.
The various prior methods are difficult to apply to the measurement of surfaces that are not approximately normal to the measurement direction because they focus on relative motions of the measured object in that normal direction. Thus, the various techniques are not well suited to the measurement of the surface shape of non-flat objects such as logs. The present invention extends the scope of method described in U.S. Pat. No. 7,003,894 to enable effective surface shape measurements of such non-flat objects.